John B. Deaton

Generation of ultrasound by repetitively Q-switching a pulsed Nd:YAG laser

Repetitively Q-switching a Nd:YAG laser during a single flashlamp pulse has been used successfully to generate a train of acoustic pulses with a repetition rate as high as 53 kHz. The spectral content of this multiple-pulse ultrasonic signal is significantly narrower in bandwidth than that of a single pulse. A corresponding reduction in the detection system bandwidth results in a marked improvement in detection sensitivity.

Generation of narrow‐band ultrasound with a long cavity mode‐locked Nd:YAG laser

A passively mode‐locked, flashlamp‐pumped Nd:YAG laser with a cavity length of 11.19 m has been developed to study the noncontact generation of narrow‐band ultrasound. The individual mode‐locked pulses acted as separate sources of ultrasound, producing a train of acoustic pulses with a repetition rate of about 13.4 MHz. The ultrasound was generated in an aluminum sample and remotely detected with a path stabilized Michelson interferometer. The energy in the multiple pulse acoustic signal was confined to a considerably reduced spectral range compared with that in a single pulse.

Dual-beam interferometer for the accurate determination of surface-wave velocity.

A novel dual-beam interferometer has been designed and constructed that enables two beams from a He-Ne laser to probe remotely the surface of a material. The separation of the two He-Ne beams is adjustable in the 15-to- 40-mm range with a spatial resolution of 2 microm. Surface-acoustic-wave measurements have been performed with two different probe separations so that the travel time for the surface waves over a known distance can be determined accurately. With the aid of autocorrelation algorithms, the Rayleigh pulse velocity on 7075-T651 aluminum has been measured to be 2888 +/- 4 m/s. The current precision of the system is limited mainly by the 10-ns sampling rate of the digital oscilloscope used. Rayleigh pulse interactions with a surface-breaking slot, machined to a nominal depth of 0.5 mm, have also been examined and the depth estimated ultrasonically to be 0.49 +/- 0.02 mm. The system may also provide a technique for direct quantitative studies of surface-wave attenuation.

Variable-cavity-length mode-locked Nd:YAG laser for noncontact generation and spectral control of narrow-band ultrasound

A passively mode-locked, flash-lamp-pumped long-cavity Nd:YAG laser was developed with a cavity length that was variable in discrete increments from approximately 11 to 60 m, permitting adjustment of the mode-locked pulse repetition frequency over a range from 13 to 2.5 MHz. Multiple-pulse acoustic signals were generated with this laser in an aluminum sample and remotely detected by a path-stabilized Michelson interferometer. The energy in the multiple-pulse acoustic signal was confined to a considerably reduced spectral range compared with that in a single pulse. Successful laser generation of spectrally selective narrow-band ultrasound presents new opportunities to integrate advanced signal-processing strategies with interferometric detection to enhance the sensitivity of laser ultrasonics for industrial applications.

Effects of laser source parameters on the generation of narrow band and directed laser ultrasound

The successful application of laser techniques for ultrasonic testing depends on the efficient coupling of optical energy into elastic energy so that laser probe detection sensitivity may be maximized. Through optimization of the laser source which is used to generate ultrasonic waves, the overall performance of laser ultrasonic systems may be enhanced by improving the efficiency with which optical energy is converted to elastic energy. This optimization depends primarily on the source laser wavelength which governs the physical interaction of the optical energy with the material of interest. For a given laser source wavelength, several techniques have been demonstrated which modify the laser source to enhance the detectability of laser ultrasonic waves and include the repetitively pulsed laser source [1,2], or temporal array, and the phased array laser source [3],or phased array. These techniques directly address the wave detectability issue by controlling the amplitude and/or the frequency content of the laser ultrasonic wave. Even though the overall conversion efficiency of optical energy to elastic energy is not improved primarily by repetitive pulsing or phasing laser arrays, the detectability of a given laser ultrasonic wave may be enhanced beyond that obtained using a single laser source.

Laser ultrasonics: generation and detection considerations for improved signal-to-noise ratio

It is shown that improvement in the detection sensitivity of laser-ultrasonic systems may be obtained by generating narrowband acoustic signals using both temporal and spatial modulation of the generating laser. A laser-generated acoustic tone burst waveform will have lower peak amplitudes than a single acoustic pulse providing the same system SNR. Consequently, lower power density laser pulses may be used to avoid surface damage.

Laser Generation of “Directed” Ultrasound in Solids Using Spatial and Temporal Beam Modulation

Laser based methods for generation and detection of ultrasound are well established laboratory tools[1]. Since only beams of light interact with the surface of an object, laser ultrasonic methods are potentially non-contacting and remote and may be used in applications involving hazardous environments or unusual component geometries. However, for use in the field as a nondestructive testing tool, or in the factory as a sensor for process control, laser ultrasonic methods suffer by comparison with more conventional contact transducer techniques with regard to their generation efficiency and sensitivity. In an effort to improve the overall sensitivity of laser ultrasonic systems, schemes for temporally and spatially modulating the laser generation source have been investigated.

Laser Generation of Narrow Band Ultrasound

Laser based sensor systems to replace conventional piezoelectric contact transducers for ultrasonic testing continue under development for applications where contact with the specimen surface is undesirable or impossible. To date, such systems are considerably less sensitive than their piezoelectric counterparts. As a result, a great deal of effort has contributed to the development of a number of interferoroetric transducer systems to detect ultrasound. Increasingly, however, researchers have begun looking at laser ultrasonic sources to see what improvements might be made to enhance overall system sensitivity for laser generation and detection of ultrasound.

LASER SOURCE PARAMETER SELECTION AND ARRAY CONSIDERATIONS FOR ENHANCING DETECTION SENSITIVITY IN LASER ULTRASONIC SYSTEMS

Recent work has demonstrated that significant gains can be achieved in laser ultrasonic signal generation efficiency and detection sensitivity by controlling the temporal and spatial source parameters as well as power density. In particular, temporal and spatial (array) modulation has been used to generate narrow band (toneburst) and directed ultrasound, thereby permitting useful reductions in electronic bandwidth of the detection system. The narrow band nature of the resulting ultrasonic signal permits filtering of the optically received waveform with dramatic improvements in signal-to-noise ratio in some cases. A numerical processing algorithm has been developed to permit modeling of the thermoelastic and ablative regimes for laser sources of finite extent. This model has been used to help predict the effects of altering source and array parameters with very good agreement with experimental results.

Laser Generation and Interferometric Detection of Ultrasound in Anisotropic Materials

Current investigation into the influences of material anisotropy on the signature of laser generated waveforms in single crystals using interferometric detection has shown that the orientation of the lattice not only influences the speed of wave propagation but also the overall shape of the detected waveform. In few grained crystalline materials which exhibit strong anisotropy, the misorientation of grains relative to one another may be observed in the signature of detected waveforms. As a result of the reproducibility and the inherent directivity of laser generated ultrasound, the lattice orientation of a specimen relative to its boundaries may be inferred by observing both wave velocities and wave signatures.